The present invention is directed to methods and apparatus for joining multilayer joints of pipe. Dual layer and multilayer pipe joints are typically used in specialty pipeline systems, where pipeline repair or replacement is costly and corrosive elements or abrasive pipeline fluids substantially decrease the useful life of typical metal pipes. For instance, dual and multilayer pipelines are typically installed in subsea operations and in pipelines intended for transportation of corrosive or abrasive materials.
Although various barrier systems and reverse current techniques have been used to extend the life of the metal pipelines, pipelines having an inner material in contact with the fluid have been most widely accepted in the industry. Moreover, pipelines formed from only plastic materials have not been widely accepted in large pipeline systems because of poor stress characteristics. On the other hand, piping systems constructed of both metal and plastic layers have been widely accepted within the process piping industry.
A dual layer pipe typically comprises an outer casing which provides bending, tensile and radial strength to the pipe, and an inner plastic or rubber liner which serves as a smooth noncorrosive flow surface. Trilayer pipe typically utilizes a similar outer steel casing and an inner plastic pipe, but an annulus is provided between these layers which is typically filled with a cement grout or other inert material. Pipe with more than three distinct structural layers is also possible, although less common. For the purposes of the present invention, multilayer pipe is defined as pipe with three or more distinct structural layers.
A multilayer pipeline has the same benefit as a dual layer pipeline, but offers additional protection to withstand higher degrees of pressure, shock, impact and cyclic loading. Also, dual layer pipe may be suited for process piping applications, but is not suitable for many pipeline installations (e.g., subterranean applications). Multilayer pipe, on the other hand, is well suited for both process piping applications and pipeline applications. The cement grout provides a convenient means of joining together the outer steel casing and the inner plastic pipe. Further, the cement grout layer enables the multilayer pipeline to better withstand corrosion deterioration of the metal casing, serves as an added barrier between the metal layer and plastic layer to inhibit corrosion of the steel pipe if the plastic pipe were to leak (thereby extending the effective life of a pipeline handling corrosive fluids), serves as an insulator to reduce temperature variations in the transported fluids, and provides a means for regulating the pipeline buoyancy by varying the density of the cement grout mixture.
Corrosion resistant pipe may be formed by painting or coating the interior of a metal pipe. Dual layer pipe, on the other hand, is generally defined as pipe having two layers of distinct structural strength, and may be formed by extruding steel over plastic pipe, by contracting plastic pipe and subsequently expanding the plastic pipe to the interior of a metal pipe, or by wrapping steel bands over plastic pipe. Although dual layer pipe has been used commercially for years, it does not offer the substantial benefits of a multilayer pipe, and is seldom used in the pipeline field. Moreover, industrial acceptance of dual layer pipelines has been limited, in part, because the methods and apparatus for joining dual layer pipe sections have been time consuming and have often not resulted in leakproof seals between the plastic and metal layers of a pipe, especially at higher pressures.
Various types of joints for joining sections of pipe are depicted and described by Robert H. Perry and Cecil H. Chilton in Chemical Engineers' Handbook, Title Edition, commencing at page 6-57. Non-metallic pipe and lined pipe systems, and joints typically used in these systems, are subsequently described at page 6-79. Pipeline joints are also depicted in U.S. Pat. Nos. 3,827,733; 3,986,731; 4,011,652; 4,053,247; and 4,060,263. Yet, none of these joints have proven to be satisfactory in many situations, either because of pressure or expense.
Threaded pipeline joints are not generally accepted because they do not provide a continuous integral wall, and are therefore prohibited in many underground pipeline applications. Threaded joints also provide stress and corrosion concentration points, and do not lend themselves well to dual or multilayer pipe joints. Other joints do not adequately seal the plastic liner and allow corrosive fluid to come in contact with the metal outer casing. Dow Chemical Corporation and Peabody Corporation supply a dual layer plastic lined pipe, but the sections are flanged with molded raised face ends, or with ends suitable for gasketed pipe joints. These pipe sections are thus expensive and time consuming to install. Moreover, special precautions must be taken to insure that no welding operations are done on the pipe or flange components, since excessive heat can cause liner decomposition and failure.
For barrier corrosion control, as in coated pipe, a bell and sleeve joint supplied by AMF Tuboscope is also commercially available for joining pipe sections. This technique, however, requires that each end of pipe section be flared outwardly or belled to allow insertion of a joining sleeve, which substantially increases the cost of the pipe sections. A thin sleeve with an epoxy coating is provided for joining the sections of the pipe, but care must be taken so that the weld does not contact the sleeve when the pipe sections are joined or the epoxy coating may be severely damaged.
The above-described joints do not efficiently and reliably function to join sections of dual layer pipe, and these above-described joints therefore limit the acceptance of dual layer pipeline systems. Moveover none of the above-described joints may be satisfactorily employed to join sections of a steel-cement-plastic layer pipe, as described above. Although multilayer pipe is widely recognized as obtaining the same benefits as dual layer pipe plus significant additional features, the absence of an efficient and reliable multilayer pipe joint limits the industrial use and acceptance of standardized joints of multilayer pipe.
U.S. Pat. No. 3,662,045 describes a method for providing a multilayer pipeline which had proven satisfactory in many applications. The technique described in this patent, however, is particularly suitable for repairing a conventional metallic pipeline by inserting a smaller diameter plastic pipe within the line and subsequently filling the annulus with a cement grout. More particularly, the annulus of the multilayer pipeline described in this patent is filled with cement once the metallic line and inner plastic pipe are in place. This technique does not utilize a prefabricated joint for joining multilayer pipe sections, but rather forms a multilayer pipe in the field and uses flanged or welded joints spaced thousands of feet apart to join sections of pipe, wherein the joint is also formed at the installation site by filling the annulus portion with the cement grout. Thus, the technique described in this patent is not adaptable for forming convenient lengths of multilayer pipe at a plant location and transporting multilayer pipe sections to required installation sites.
The present invention overcomes these problems by providing a multilayer pipe joint which can be easily, effectively and reliably utilized to join prefabricated sections of multilayer pipe at the installation site. This enables convenient length (e.g. 40 foot) of multilayer pipe to be completely formed at a manufacturing plant with a cement grout in place, and the pipe sections may then be joined at the installation site without the need for time consuming cement pumping procedures.
The disadvantages of the prior art are thus overcome with the present invention, and novel methods and apparatus are hereinafter described for efficiently and reliably joining together sections of a multilayer pipeline.